The C8 aromatics exist as ethylbenzene and three isomers of xylene (dimethylbenzenes) which are separated only with great difficulty because they have boiling points which are very close together. Para-xylene is used in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers, such as polyester. Meta-xylene is used for the manufacture of insecticides, isophthalic acid or alkyd resins. Ortho-xylene can be used as material for plasticizers. Benzene di- and tri-carboxylic acids have wide industrial application including the manufacture of polyesters, polyamides, and fibers and films. For commercial manufacture of these products the required source of high purity benzene di- and tri-carboxylic acids may be obtained from a corresponding substituted aromatic compound by catalytic oxidation of methyl moieties to carboxylic acid moieties, advantageously in a liquid-phase medium.
The possibility of utilizing a liquid-phase instead of vapor-phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by variable valence metals, especially cobalt, in a liquid-phase of saturated lower aliphatic acid at elevated temperature and pressures to maintain the liquid phase of the aliphatic acid. Combinations of cobalt and manganese with a source of bromine have become preferred for commercial use, for example, see U.S. Pat. No. 2,833,816. A key element in obtaining benzene di- and tri-carboxylic acids having suitable high purity for these oxidation processes is using an oxidation feedstock of high purity.
Various commercial processes for separation of para-xylene from C8 aromatics have been developed as alternatives to fractional distillation. Such processes utilize either differences of the freezing points between ethylbenzene, ortho-xylene, meta-xylene and para-xylene, (fractional crystallization) and/or processes based upon properties of zeolite materials to selectively adsorb para-xylene from mixtures of C8 aromatics; the adsorbed paraxylene is recovered after desorbing from the zeolite.
Either of these processes can recover paraxylene in high yields from available mixtures of C8 aromatics. However, they involve reprocessing large amounts of the resulting filtrate from the crystallization process or the raffinate from the adsorption process. These streams are depleted in paraxylene and contain relatively high proportions of ethylbenzene, ortho-xylene, and meta-xylene. Furthermore, these streams are typically subjected to further processing downstream of the crystallization or adsorption process.
Crystallization methods have been used in commercial processes to separate para-xylene from aromatic starting materials containing ethylbenzene as well as the three xylene isomers. Use is made of the fact that melting point temperatures of the individual isomers of xylene are significantly different. Ortho-xylene has a freezing point of negative 25.2° C., meta-xylene has a freezing point of negative 47.9° C. and para-xylene has a freezing point of 13.3° C. However, conventional crystallization methods can be used to make para-xylene with a purity of 99.8 percent by weight only with great expense.
Crystallization processes to recover para-xylene from a mixture of C8 aromatics requires cooling a feed stream, for example an equilibrium mixture of isomers from reformate and/or xylene isomerization processes. Because it's melting point is much higher than that of other C8 aromatics, para-xylene crystals are readily formed in a crystallizer after refrigeration of the feed solution. Feed mixtures of refinery aromatic streams typically contain about 22 to about 23 percent by weight of para-xylene. In order to crystallize a substantial amount of the para-xylene from solution, the solution has to be cooled to just above the eutectic temperature (i.e. the temperature at which a second component start to co-crystallize and contaminate the para-xylene crystals). The eutectic temperature is determined by the composition of the remaining mother liquor after para-xylene crystals are removed from the mixture (mostly meta-xylene, ortho-xylene and ethylbenzene). Although not known with absolute certainty, it is believed that the eutectic temperature decreases with higher relative composition of ethylbenzene in the remaining mother liquor. As the eutectic temperature decreases, the concentration of para-xylene in the outlet stream also decreases, increasing the para-xylene recovery. Given a mixture of xylenes with relative ratio of para-xylene:meta-xylene:ortho-xylene:ethylbenzene of about 2:4:2:1, at temperatures within about 3° to 6° C. of the eutectic temperature the para-xylene recovery is limited to about 70 percent.
Typically a reject stream depleted in para-xylene, but containing relatively high proportions of ethylbenzene, ortho-xylene, and meta-xylene from the crystallization process or the adsorption processes, are treated in an isomerization process which is used to increase the proportion of para-xylene in para-xylene depleted streams. The para-xylene depleted stream can be contacted with an isomerization catalyst under appropriate temperature and pressure which results in the conversion of some of the ortho-xylene and meta-xylene to para-xylene. It is also usually desirable to convert some of the ethylbenzene to prevent it from building up to high concentrations. Known catalysts are selected to enable conversion of ethylbenzene to benzene and ethane via hydro dealkylation, and/or C8 isomerization to an equilibrium mixture of xylenes.
Processes for making para-xylene therefore included combinations of isomerization with fractional crystallization and/or adsorption separation. The disadvantage with such combinations is that, despite improvements in catalyst performance isomerization technology is only able to produce equilibrium or near-equilibrium mixtures of the xylene isomers and typically is also relatively inefficient for the conversion of ethylbenzene to benzene and/or isomers of xylene. Consequently big recycles of the reject streams back through these processes are needed to ensure the conversion of the C8 aromatics stream to para-xylene is maximized with or without the additional recovery if desired of ortho-xylene and/or meta-xylene. There is a need therefore for improved processes and chemical plants for the production of para-xylene from mixtures of C8 aromatics, which in particular address the problems associated with large recycles and/or low ethylbenzene conversions.
Accordingly, it is an object of the invention to overcome one or more of the problems described above.
A new approach to recovery of a very pure aromatic isomer has now been found when processing aromatic starting materials, for example, a pure para-xylene product from liquid mixtures containing ethylbenzene as well as the three xylene isomers. The new approach beneficially provides a process for recovering para-xylene having a purity of at least 99.5 percent by weight, and advantageously 99.8 percent from liquid mixtures of aromatic compounds, even containing ethylbenzene as well as the three xylene isomers.
As will be described in greater detail hereinafter, the present invention provides processes for recovery of purified products from a fluid mixture by means of an integrated fractional crystallization and perm-selective membrane separation apparatus. Integrated apparatus of the invention comprises a fractional crystallization unit coupled to one or more devices using polymeric membranes for recovery of purified products.
There is a need for a cost effective method of producing high purity para-xylene from a C8 aromatic mixture containing para-xylene, meta-xylene, ortho-xylene and ethylbenzene which overcomes the aforementioned eutectic limit for crystallization.